The present invention relates to the field of superabsorbent powders for the absorption of aqueous fluids which are of value in particular for the preparation of articles of hygiene intended to retain body fluids.
These powders, which are experiencing very rapid industrial development, are characterized, for their application, by three main types of properties: their absorption properties, the mechanical properties of the gels which they form by absorption of aqueous fluids, and what may be called the transportation properties of these gels.
The absorption properties, the advantage of which is immediately obvious in the case of absorbent products and even more so superabsorbent products, are essentially the intrinsic absorption and retention capacities, for example for saline water, and the gel setting time.
The mechanical properties of the aqueous superabsorbent gels are related to the need for articles of hygiene to be able to absorb aqueous fluids when they are in a situation of compression or simply when they contain a large amount of superabsorbent powder. In this instance, there has been particular interest in systems with a high concentration of superabsorbent powder which exhibit a high capillary suction capacity under pressure.
The transportation properties of the superabsorbent gel are related to the ability of the fluids to diffuse rapidly into the superabsorbent and not to be blocked by the gel which is formed on first contact (gel blocking). These blocking phenomena are reflected by leakages of liquids which have not succeeded in crossing a gel barrier. The importance of them is estimated by measuring the porosity under certain conditions of pressure and of impregnation of the superabsorbent by the aqueous fluid. This porosity is obviously dependent on the geometry (particle size) of the powder but also on the mechanical properties of the gel, it being understood that impermeability can be an unfortunate consequence of the crushing of the gel. This is the reason why, in assessing the transportation properties of the gel, the porosity and the mechanical modulus of the superabsorbent gel are often taken for each other, the latter also being related to the capillary suction mentioned above.
In order to assess these properties, the measurements indicated below are usually carried out.
The intrinsic absorption capacity of the superabsorbent resin is obtained by the so-called xe2x80x9ctea-bagxe2x80x9d test (measurement of the absorption and retention capacities by the xe2x80x9ctea-bagxe2x80x9d method, EDANA, European Association of Nonwovens), by which test the water uptake is measured by weight with respect to the weight of a dry resin enclosed in a heat-sealable paper bag, after immersion for about 20 minutes in an aqueous solution containing 0.9% of sodium chloride and draining for about ten minutes; the same bag is subsequently centrifuged for three minutes at a specified speed and weighed again, which gives the saline water retention capacity.
The capillary suction capacity under pressure (CS) is the measurement of the ability of a bed of 1.5 g of superabsorbent product to absorb water containing 0.9 g of salt per liter under a load, here chosen as 5 kPa. This is a well-known property, designated under the terms capillary absorption or suction under pressure or under load (the term xe2x80x9cabsorption under loadxe2x80x9d, shortened to AUL, is frequently encountered). The procedure thereof is universally practised. A description of it will be found, for example, in Patent Application EP-A1-0,258,120.
In order to measure the porosity of a bed of gel, 4 g of superabsorbent resin are swollen with 140 g of a 0.9% by weight aqueous sodium chloride solution. The gel formed is placed in a container with a diameter of 52 mm and a height of 55 mm. The container is subsequently connected to the line of a tank containing the saline solution (0.9% NaCl) and the amount of solution which manages to pass through the gel is continuously weighed. The pressure of the liquid passing through the bed (2.5 kPa) is kept constant by controlling the level of the feed tank. The mass of solution collected after 10 minutes is regarded as a measure of the porosity of the gel.
The gel setting time is measured as follows. 3 g of superabsorbent powder are poured into a 250 ml beaker with an external diameter of 60 mm. A magnetic stirrer with dimensions 48xc3x978 mm is placed inside the beaker and the entire assembly is placed on a magnetic stirrer plate. Stirring is adjusted to 600 revolutions per minute immediately before pouring 100 ml of an aqueous solution containing 0.9% of salt into the beaker and simultaneously starting a stopwatch. The latter is stopped at the moment when the stirring vortex disappears. The time in seconds shown by the stopwatch corresponds to the gel setting time of the product.
The problem posed to the person skilled in the art is that absorption and mechanical properties are conflicting quantities and that any attempt to increase, by synthesis, the intrinsic absorption capacity level of superabsorbent polymers results in a decline in the mechanical properties, in particular in the capillary suction. The preparation of a superabsorbent powder exhibiting simultaneously
a high intrinsic absorption capacity,
a high retention,
a high capillary suction,
a high porosity and
a short gel setting time,
appears as an impossible compromise, which there is no hope of achieving either by direct synthesis or by formulation, since it would clearly be expected that a mixture of powders with different properties would only correspond to a product with mediocre intermediate properties.